With the patch 8.2 Azeroth opened the gate of a new pet battle dungeon: Stratholme.
To start this journey head to Radek Fuselock for the horde side or Tizzy Gearjolt if you're from the alliance.
Reports of sinister machinations lead us to Sean Wilkers, a paladin who keeps company to us during this expedition and later will function as this dungeon's pet vendor.

Your first playthrough grants an Ultimate Battle-Training Stone as usual, and the following runs in the Challenge Mode reward the new currency Cleansed Remains which is used to buy pets. Note that these pets are not tradable, so you have to work for them.

Furthermore as a novelty to pet battle dungeons there are two special bosses within Stratholme: Fras Siabi and Postmaster Malown.

1. Fras Siabi

works like a rare boss in the form of a world quest, meaning that there is a low chance for him to spawn randomly. He appears in stage 7 of the dungeon, right behind the Risen Guard and beating him awards an additional Cleansed Remains. However if you are really eager to get this currency you can leave the dungeon prematurely and progress to stage 7 again until he spawns. But be warned: he is a tough opponent!

To fight

2. Postmaster Malown

you first have to make him spawn. In order to do that find and right-click a letter that is laying hidden inside the dungeon. Once you've done that go the mailbox at 57.8 13.6 (in the vicinity of Nefarious Terry) and he'll appear. Now you have 20 minutes to fight him. After that time Malown will disappear. Beating him rewards you with the achievement Malowned and the pet Burnout.

There are multiple points where the letter can spawn and some of them might still be undiscovered. If you happen to find one that isn't listed in this guide yet please let us know with the coordinates (and a screenshot if you don't mind).
If you are using TomTom you can copy those coordinates to find the letter:

You know if you get Fras encounter in your run straight after defeating the critters in the trap with the gates. An emote in red text will appear and you can just run past Risen Guard and the fountain, and fight Fras.

I started late on this one... wanted to grind pet levels for backup just in case. First clear I found the letter after clearing the scenario and going back. Second week I managed to get lucky with Fras. I'm wondering though if it's possible to encounter both of the bonus encounter bosses in the same scenario?

Yes, it's definitely possible to get both Malown and Fras Siabi in the same run. I got both, the first time I went in....and thanks to the pain others went through before me (those who have created strats here), I was able to beat them both.

FYI, if you "teleport out of dungeon" the whole thing resets. if you select "leave instance group" you get a prompt about not being able to get back in. i guess they don't want people to try to spawn Frias.

Made 2 trips around the perimeter of the dungeon as well as the cart in the middle zoomed all the way in checking between every crack and crevice by every crate and barrel. No letter. I think I'm one of the unlucky ones. I'm sure if I reset the dungeon I might find it, but I don't think the letter is there every time.